1. Technical Field
The present invention relates to an apparatus that drives a delivery roller and a paper feed roller to convey a continuous paper web pulled off a paper roll, for example, to a printing position for printing while adjusting the paper tension by a tension damping mechanism. The invention also relates to a paper transportation control method for an apparatus that can control the paper supply operation of the delivery roller that assists the paper feed operation of the paper feed roller.
2. Description of Related Art
Roll paper, which is a continuous web of paper wound into a roll, is used in roll paper printers for printing receipts, for example. The roll paper is stored freely rotatably in a roll paper compartment, and a paper feed roller conveys the paper pulled off the paper roll to a printing position for printing by a print head at the printing position. If the paper transportation load varies greatly at this time, the paper may not be conveyed at a uniform rate or the paper may become skewed during paper transportation by the paper feed roller, and such transportation problems can result in a drop in print quality. As a result, the part of the paper between the paper roll and the paper feed roller may therefore be routed around a tension damping mechanism that is urged by an urging member to maintain a specific tension on the paper while absorbing (damping) variation in the transportation load by displacement of the damping mechanism.
The transportation load on the paper feed roller increases as the size of the paper roll increases. The inertia of the paper roll results in a significant difference in the transportation load between when transportation starts, during transportation, and when paper transportation stops. There is also a significant difference in the paper transportation load between when a new roll is loaded and used for the first time, and when the roll is nearly depleted and the roll diameter is close to the size of the core. The paper transportation roller alone can therefore not sufficiently suppress deviation in the uniformity of paper transportation, and some printers therefore also have a delivery roller to assist paper transportation by pulling the paper off the paper roll. Japanese Unexamined Patent Appl. Pub. JP-A-2007-203563 and Japanese Unexamined Patent Appl. Pub. JP-A-2007-203564 teach a roll paper supply mechanism with this type of delivery roller.
Driving the delivery roller can be controlled using sensors to monitor the paper tension. The sensor can simply detect the position of the tension damping mechanism, which is displaced by the paper tension. If the tension exceeds an upper tension limit, the paper delivery operation of the delivery roller starts to assist paper transportation by the paper transportation roller, and the delivery operation stops when the paper tension returns to or below the upper tension limit.
Serial printers with a carriage operate by printing one line while paper transportation is stopped and then advancing the paper one line before stopping again and printing the next line, and the paper tension (paper transportation load, also referred as paper feed load) is therefore constantly changing as a result of thus intermittently conveying the paper.